This invention relates to the field of silica films and the preparation thereof and, more particularly, to preparation of thin silica films with controlled thickness and tunable refractive index.
Porous silica films are currently of technological interest in applications ranging from low-dielectric constant (xe2x80x9clow-kxe2x80x9d) materials (see e.g. Prakash, S. S.; Brinker, C. J.; Hurd, A. J.; Rao, S. M. Nature 1995, 374, 439; Prakash, S. S.; Brinker, C. J.; Hurd, A. J. J. Non-Cryst. Solids 1995, 190, 264; Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548; Zhao, D.; Yang, P.; Melosh, N.; Feng, J.; Chmelka, B. F.; Stucky, G. D. Adv. Mater. 1998, 10, 1380; Hguyen, C.; Hawker, C. J.; Miller, R. D.; Huang, E.; Hedrick, J. L.; Gauderon, R.; Hilborn, J. G. Macromolecules 2000, 33, 4281; Nguyen, C. V.; Carter, K. R.; Hawker, C. J.; Hedrick, J. L.; Jaffe, R. L.; Miller, R. D.; Remenar, J. F.; Rhee, H.-W.; Rice, P. M.; Toney, M. F.; Trollsas, M.; Yoon, D. Y. Chem. Mater. 1999, 11, 3080; Hendrick, J. L.; Miller, R. D.; Hawker, C. J.; Carter, K. R.; Volksen, W.; Yoon, D. Y.; Trollsas, M. Adv. Mater. 1998, 10, 1049; Doshi, D. A.; Huesing, N. K.; Lu, M.; Fan, H.; Lu, Y.; Simmons-Potter, K.; Potter Jr., B. G.; Hurd, A. J.; Brinker, C. J. Science 2000, 290, 107; Baskaran, S.; Liu, J.; Domansky, K.; Kohler, N.; Xiahong, L.; Coyle, C.; Fryxell, G. E.; Thevuthasan, S.; Williford, R. E. Adv. Mater. 2000, 12, 291; Jin, C. M.; Luttmar, J. D.; Smith, D. M.; Ramos, T. A. Mater. Res. Soc. Bull. 1997, 22, 39; Nguyen, S. V. IBM J. Res. Dev. 1999, 43, 109) to anti-reflective coatings (see e.g. Uhlmann, D. R.; Suratwala, T.; Davidson, K.; Boulton, J. M.; Toewee, G. J. Non-Cryst. Solids 1997, 218, 113; Martinu, L.; Poitras, D. J. Vac. Sci. Technol. A 2000, 18, 2619) to molecular separations (see e.g. de Vos, R. M.; Maier, W. F.; Verweij, H. J. Membr. Sci. 1999, 158, 277; de Vos, R. M.; Verweij, H. J. Membr. Sci. 1998, 143, 37; de Vos, R. M.; Verweij, H. Science 1998, 279, 1710; Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. Nature 1996, 381, 589; Lu, Y.; Ganguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W.; Guo, Y.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I. Nature 1997, 389, 364; Cot, L.; Ayral, A.; Durand, J.; Guizard, C.; Hovnanian, N.; Julbe, A.; Larbot, A. Solid State Sci. 2000, 2, 313). Standard routes to these materials include the chemical vapor deposition (CVD) of silicon oxide precursors (see e.g. Martinu, L.; Poitras, D. J. Vac. Sci. Technol. A 2000, 18, 2619); and the spin- or dip-coating of partially hydrolyzed silicon alkoxide solutions, i.e., sol-gel chemistry.(see e.g. Binker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic Press: New York, 1990). Depending on the processing conditions used in sol-gel chemistry, the resulting film can be allowed to condense during removal of the solvent to form a material of low to medium porosity (xerogel), or the pore liquid can be extracted by supercritical drying to produce porosities of up to 99.9% (aerogel). Recently, it has been shown that by modifying the gel with organosilanes prior to solvent removal, aerogels can also be formed without the need for supercritical drying (see e.g. Prakash, S. S.; Brinker, C. J.; Hurd, A. J.; Rao, S. M. Nature 1995, 374, 439. Prakash, S. S.; Brinker, C. J.; Hurd, A. J. J. Non-Cryst. Solids 1995, 190, 264).
It is among the objects of the present invention to produce silica films with good control over film porosity and film thickness. This simultaneous control of both parameters is important because it fully defines the optical pathlength within the films.
An embodiment of the method hereof for forming porous films relies on the well-established behavior of polyionic species to adsorb readily onto a surface of opposite charge, reported initially by Iler in the mid 1960s, (see e.g. Iler, R. K. J. Colloid Interface Sci. 1966, 21, 569) but attracting a great deal of interest more recently (see e.g. Kleinfeld, E. R.; Ferguson, G. S. Science 1944, 265, 370; Decher, G.; Hong, J. D. Ber. Bunsenges. Phys. Chem. 1991, 95, 1430. Keller, S. W.; Kim, H.-N.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 8817. For a review of stepwise multilayer self-assembly techniques, see: Decher, G. Science 1997, 277, 1232). Oligomeric silica species can be adsorbed from sols derived from tetraethylorthosilicate (TEOS) onto substrates bearing a very thin (xcx9c6 xc3x85) layer of positively charged polyelectrolyte. Furthermore, by varying the conditions used to prepare the sol, robust films can be prepared with refractive indices in the range of 1.16-1.31. Although the prior art includes reported adsorptions of colloidal silica, (see e.g. Iler, R. K. J. Colloid Interface Sci. 1966, 21, 569; Lvov, Y. M.; Rusling, J. F.; Thomsen, D. L.; Papadimitrikopoulos, F.; Kawakami, T.; Kunitake, T. Chem. Commun. 1998, 1229. Lvov, Y.; Ariga, K.; Onda, M.; Ichinose, I.; Kunitake, T. Langmuir 1997, 13, 6195. Ariga, K.; Lvov, Y.; Onda, M.; Ichinose, I.; Kunitake, T. Chem. Lett. 1997, 125) the method hereof allows unprecedented control of the amount of porosity within these films.
In accordance with an embodiment hereof, a method of forming a porous silica film is set forth comprising the following steps: a) providing a substrate; b) coating, on a surface of the substrate, a layer of charged polyelectrolyte; and c) applying an aged silica-bearing non-colloidal solution to the coated surface of the substrate to adsorb porous silica thereon. The steps (b) and (c) together comprise an adsorption cycle, and the adsorption cycle is repeated a number of times to control film thickness. In a preferred embodiment of the invention, the silica-bearing solution is an aqueous solution of TEOS and a base. In this embodiment the step (b) of coating on a surface of the substrate a layer of charged polyelectrolyte comprises applying to the surface an aqueous solution of PDDA. The age and concentration of the silica-bearing solution are selected to control the porosity and the index of refraction of the porous silica film.
Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.